Nerve cells communicate by conducting electrical signals along slender cytoplasmic extensions known as axons. Animals have evolved two basic mechanisms for increasing axonal conduction velocity. One is to increase axonal diameter and the other is to insulate axons by a process called myelination, which is a tight spiral wrapping of the axons that is formed by myelinating cells. In vertebrates the growth of axon diameter is caused principally by the accumulation of space-filling cytoskeletal polymers called neurofilaments inside the axons, and this is regulated locally by chemical signals from the myelinating cells. It is known that neurofilaments are transported along axons and that they alternate between rapid movements and prolonged pauses. The proportion of the time that the neurofilaments spend pausing is likely to be a principal determinant of their residence time in axons. This is a collaborative experimental and modeling project involving a biologist at Ohio State University and a physicist at Ohio University. The central hypothesis to be tested is that myelinating cells control axonal caliber by regulating neurofilament pausing. A computational model will be developed that relates the moving and pausing behavior of neurofilaments to their distribution along axons. The model will be based on detailed kinetic parameters of neurofilament movement derived experimentally in cultured neurons and will be verified experimentally by fluorescence microscopy of neurofilament movement in myelinated axons in tissue culture. The proposed research will generate a rigorous and quantitative framework that relates the size and shape of axons, which is a key influence on their electrical properties, to the moving and pausing behavior of their internal constituents. The research will involve graduate and undergraduate students in both the physical and biological sciences, providing an integrated and cross-disciplinary training experience at the interface between computational and experimental biology.
This was a collaborative project between a biologist at Ohio State University (Anthony Brown) and a physicist at Ohio University (Peter Jung). A unique feature of this project was the tight integration of experimental and computational modeling approaches to test hypotheses about a fundamental problem in neuroscience. This project focused on neurofilaments, which are microscopic protein polymers in the cytoplasm of nerve cells. Nerve cells extend long cytoplasmic processes called axons, which conduct nerve impulses. Neurofilaments are space-filling architectural elements that function to maximize the cross-sectional area of axons, thereby maximizing the rate at which they propagate nerve impulses. During development, neurofilaments accumulate in axons causing them to expand. Neurofilaments are also important because they accumulate abnormally and excessively in axons in many neurodegenerative diseases. It is therefore important to understand the biology of neurofilaments. Our laboratory has discovered that neurofilaments are transported along axons in a rapid intermittent manner, which that can be likened to ‘stop and go’ traffic on a highway. We believe that nerve cells control axonal neurofilament content, and thereby axonal morphology, by regulating the number of moving neurofilaments and the velocity at which they move. We also believe that perturbations of neurofilament transport may explain the abnormal and excessive accumulation of neurofilaments that occurs in neurodegenerative diseases. Intriguingly, there is published evidence that the accumulation of neurofilaments in developing axons is influenced by myelinating cells, which are cells that wrap axons in an insulating sheath called myelin. The central goal of this project was to test the hypothesis that myelinating cells control axonal caliber by locally regulating the axonal transport of neurofilaments. To address this, we developed computational and experimental approaches to analyze the kinetics of neurofilament transport in myelinated axons in cell culture. Briefly, we used fluorescence microscopy to observe the movement and computational modeling to extract information about the velocity and pausing behavior from the experimental data. The principal outcome of these studies is the finding that neurofilament transport slows locally and reversibly in myelinated axons and that this is due to an increase in the proportion of the time that the neurofilaments spend pausing during their rapid intermittent movement along axons. As expected, this slowing of neurofilament transport correlated with a local accumulation of neurofilaments and an expansion of axonal cross-sectional area. This suggests that myelinating cells can signal locally and reversibly to axons to alter axonal morphology by regulating neurofilament pausing behavior. Thus we have established a link between neurofilament transport and axonal morphology that has important implications for axonal physiology. Future studies will examine the molecular mechanism of this intercellular signaling. The project has involved graduate and undergraduate students in a vibrant interdisciplinary training environment at the interface of physics and biology and has resulted in five publications in peer-received scientific journals as well as three others that are in preparation.
